About twenty percent of the world population suffers from various allergic diseases such as asthma, allergic rhinitis, food allergies, atopic dermatitis and anaphylaxis. The alarming increase in the prevalence of allergic diseases over the past decade has led to a clear need for more effective treatment.
The interaction between IgE and mast cells or basophils is the primary effector pathway in allergic responses. IgE binds to high-affinity receptor (FcεRI) for its constant region, found almost exclusively on the surface of these cells. The binding itself, in spite of the low dissociation rate, does not result in stimulation of the cell. However, cross-linkage of cell surface-bound IgE by multivalent antigen causes receptor aggregation, triggering explosive cellular degranulation whereby mediators of allergy such as cellular degranulation whereby mediators of allergy such as histamine and seretonin are released.
The fact that distribution of the FcεRI receptor is restricted to cells participating in an allergic response makes it an attractive candidate for targeted immunotherapy by chimeric cytotoxins. Chimeric cytotoxins are a novel class of targeted molecules constructed by gene fusion techniques. These molecules are composed of cell targeting and cell killing moieties, enabling them to recognize and distroy cells overexpressing specific receptors.
The bacterial toxin Pseudomonas exotoxin (PE) used in chimeric protein constructs, is a product of Pseudomonas aeruginosa. Having accessed the cytoplasm, PE inhibits protein synthesis by its ADP-ribosylation activity, thus causing cell death (Middlebrook, J. I., and Dorland, R. B. 1984. Bacterial toxins: cellular mechanisms of action. Microbiol. Rev. 48, 199.). Effective chimeric cytotoxins have been constructed by fusion of cDNAs encoding various growth factors or single chain antibodies with PE derivatives lacking intrinsic cell binding capacity. One of these chimeric proteins designated IL2-PE40, constructed to target and selectively eliminate activated T cells overexpressing IL2 receptors, was shown to provide effective and selective immunosuppression in various models of autoimmune disorders, graft rejection and cancer (Lorberboum-Galski, H. 1994. Interleukin 2-Pseudomonas exotoxin A (IL2-PE40) chimeric protein for targeted immunotherapy and the study of immune responses. J. Toxicol.-Toxin Rewiewes, 13 (1), 105.).
The entire recombinant constant region of IgE (Fcε) expressed in bacteria, have an affinity for FcεRI receptor comparable to that of the native IgE, as well as the capacity to sensitize basophils for anti-IgE indused histamine release. When recombinant fragments of human Fcε expressed in bacteria, were tested for receptor binding, a peptide corresponding to residues 301-376 at the junctions of domains 2 and 3 of the constant region was found to be sufficient for high-affinity binding to the receptor. It was also reported that ε-chain dimerization was not required for receptor binding (Helm, B., Marsc, P., Vercelli, D., Padlan, E., Gould, H., and Geha, R. 1988. The mast cell binding site on human immunoglobulin E. Nature 331, 180.).
The present invention generally relates to a novel approach for the therapy of allergic responses. At present the major known groups of drugs used in the treatment of asthma and allergic disorders are:                1. β2 agonists—produce airway dilatation through simulation of β2 adrenergic receptors.        2. Methylxantines—smooth muscle relaxants, produce bronchodilatation.        3. Glucocorticoids—reduce inflammation.        4. Cromolyn sodium—prevents mast cell degranulation.        5. Antihistamines—prevents histamine action on it's target cells.        
Although widely used, all of these drugs have notable disadvantages in regard to:                1. Specificity: The action of all of these drugs (except cromolyn sodium) is not mast cell specific. Therefore, they can not prevent the release of allergy mediators but rather reverse or block the effects caused by their action. The treatment by these drugs is symptomatic, it can be started only after the onset of the allergic reaction and thus can't be used in a prophylactic manner.        2. Toxicity: Being non-specific, these drugs exert their action on various tissues and organs causing serious side effects. The major side effect of β2 agonists is tremor, but they also cause cardiac arrhythmias; Methylxantines stimulate the central nervous system, causing nervousness, nausea, vomiting, anorexia, headache and cardiac muscle-causing tachycardia. At high plasma levels there is a danger of seizures and arrhythmias. Antihistamines affect the central nervous system, causing sedation. Steroids are most harmful, causing suppression of the pituitary-adrenal function, fluid and electrolyte disturbances, hypertension, hyperglycemia, increased susceptibility to infections, osteoporosis and arrest of growth in children.        3. Duration of the effect: β-adrenergic agonists, aminoxantines and antihistamines are mostly short-acting drugs, and as such must be administered frequently. Steriods which are long-acting drugs, have also long induction time and are of little value in emergencies.        
The only existing mast cell specific drug is Cromolyn sodium. This drug can be used prophilactically, essentially without side effects. However, it has a very short half life, very long induction time, it can be applied only locally and only part of the patients respond to it. All these make use of Cromolyn sodium very limited.
A number of attempts to interfere with interaction between IgE and it's high-affinity receptor, as a basis for anti-allergy therapy, have been reported in recent years. Recombinant peptides comprising structural elements from IgE (Helm, B., Kebo, D., Vercelli, D., Glovsky, M. M., Gould, H., Ishizaka, K., Geha, R., and Ishizaka, T. 1989. Blocking the passive sensatization of human mast cells and basophil granolocytes with IgE antibodies by a recombinant human ε-chain fragment of 76 amino acids. Proc. Natl. Acad. Sci. USA 86, 9465.) or FCεRI (Ra, C., Kuromitsu, S., Hirose, T., Yasuda, S., Furuichi, K., and Okumura, K. 1993. Soluble human high affinity receptor for IgE abrogates the IgE-mediated allergic reaction. Int. Immunol. 5, 47.; Haak-Frendscho, M., Ridgway, J., Shields, R., Robbins, K., Gorman, C., and Jardieu, P. 1993. Human IgE receptor a-chain IgG chimera blocks passive cutaneous anaphylaxis reaction in vivo. J. Immunol. 151, 351.) have been investigated as competitive inhibitors of the IgE-FcεRI interaction. Monoclonal antibodies generated against IgE (Baniyash, M., and Eshhar, Z. 1984. Inhibition of IgE binding to mast cells and basophils by monoclonal antibodies to murine IgE. Eur. J. Immunol. 14, 799) or FcεRI (Kitani, S., Kraft, D., Fischler, C., Mergenhagen, S. E., and siraganian, R. P. 1988. Inhibition of allergic reactions with monoclonal antibody to the high affinity IgE receptor. J. Immunol. 140, 2585.), capable of blocking IgE binding to the receptor, without causing mast cell degranulation have also been tested. However, the affinity of IgE for FcεRI is very high (KM=10−10M), so that once it is bound to it's receptor, the IgE molecule remains attached to the cell membrane for several weeks. Moreover, mast cell can be activated at low receptor occupancy: the cross-linkage of as few as 5% of receptors is sufficient to cause mast cell degranulation. These two properties of the system impede inhibition by competitive agents, thus limiting their clinical value. Our anti-allergy molecule depends to a much lesser extent on the ability to compete with IgE. Once having entered the target cell through a non-occupied IgE receptor, the chimeric protein affects the target cell. Moreover, early expression of the receptor in the maturation course of mast calls should allow the elimination of immature target cells before they are capable of mediator release. As the receptor is not expressed on stem cells, no damage to bone marrow is expected on the whole.
The IgE system is quite complex and diverse. Interactions between IgE and its binding structures have many functions apart from the allergic response, some of which are only beginning to emerge. Monoclonal antibodies against IL-4, the IL-4 receptor or the low-affinity IgE receptor eliminate expression of IgE in mice but have more general immunosupressive effects. The advantage of the present invention in which the high-affinity IgE receptor is targeted and not the overall IgE system, is therefore evident.